Gas turbines generally include a compressor, one or more combustors, a fuel injection system and a turbine. Pressurized air flows from the compressor into the combustor where the air is mixed with fuel and the gaseous mixture burns. Typically, air from the compressor is turned through ducts in the combustor back towards an annular array of cylindrical combustors. Some of the air entering the combustor is mixed with fuel in mixing zones at the upstream end of the combustor. Some air flows around the combustor to cool the lining of the combustor and some air flows into dilution zones in the combustor. The heated air and combustion gases exit the combustor through transition ducts and enter the inlet to the turbines.
Combustors used in industrial gas turbines are often required to have reduced emissions of nitrogen oxide (NOx) pollutants. The amount of NOx emissions is directly related to the combustion temperature in the gas turbine. Most efforts to reduce NOx emissions have focused on reducing combustion temperature. For example, lean, premixed combustors reduce the fuel/air ratio to less than the stoichiometric ratio. This lean fuel/air ratio reduces the peak flame temperature to much less than in an unabated diffusion flame. NOx emissions are minimized by maintaining a low flame temperature.
If the fuel/air ratio becomes excessively lean, the flame will extinguish. However, NOx emissions increase dramatically if the fuel/air ratio becomes less lean, i.e., the ratio increases. It is desirable to maintain the fuel/air ratio constant and slightly above that required to maintain combustion to minimize NOx emissions.
Lean fuel/air ratios are typically used in lean, premixed combustors, but these combustors are particularly sensitive to variations in airflow. For example, a decrease of 5% (or less) in the airflow may cause a too rich fuel/air ratio and excessive NOx emissions. Similarly, an increase of 5% (or less) in airflow may cause the fuel/air ratio to become too lean and extinguish the flame. Previous lean, premixed combustors were unable to fully modulate the airflow into the mixing zone(s) so as to maintain a constant fuel/air ratio. Accordingly, maintaining a constant fuel/air ratio has been a continuing problem for low NOx combustors.
Maintaining a constant fuel/air ratio is critical to low NOx emissions, but is difficult to accomplish. The fuel flow rate into the combustor varies by a factor of four or more over the load range of an industrial gas turbine. To hold the fuel/air ratio constant, the airflow rate must vary in tandem with the fuel flow rate. In addition, the airflow to an individual combustor and its combusting zone is sensitive to manufacturing tolerances, machine mechanical conditions, ambient temperature and component changes in the gas turbine. These factors complicate the control of airflow to an individual combustor needed to maintain a constant fuel/air ratio. Airflow control becomes more complicated when several combustors are used together, as is common in industrial gas turbines.
There are several different techniques that have been used to allow lean, premixed combustors to operate over the entire load range in industrial gas turbines. One example is fuel staging which is shown in FIGS. 1A to 1D. With fuel staging in the combustor 1, the combustion liner 2 is divided into an upstream primary zone 3 and a downstream flame holding zone 4. In the embodiment shown here flame holding is accomplished with a venturi 5, but there are other ways to achieve flame holding. To ignite the combustor (and for operation up to about 20% load of the gas turbine) fuel is injected into the combustor through nozzles 6 at the upstream end of the primary zone 3, as is shown in FIG. 1A. During this start-up phase, the fuel/air ratio is near the stoichiometric ratio and the fuel/air mixture combusts in the primary zone to produce a diffusion flame.
As shown in FIG. 1B, when the load on the gas turbine increases, about 30% of the fuel is introduced through a central nozzle 7 into the venturi flame holder and downstream of the primary zone 3 where the air and fuel from nozzles 6 mix. The amount of fuel injected through the central nozzle into the flame holder is gradually increased, while the amount of fuel injected into the primary zone is gradually reduced to starve the flame in the primary zone. As shown in FIG. 1C, to extinguish the flame in the primary zone, no fuel enters the primary zone through nozzles 6 and all of the fuel is directly injected into the flame holder zone. As shown in FIG. 1D, after the flame in the primary zone is extinguished, fuel is again injected into the primary zone for mixing with air, but the fuel/air mixture does not combust until reaching the venturi flame holder zone 4. A small portion of fuel, about 17%, flows through nozzle 50 to maintain a rich fuel/air mixture at the flame kernel at the start of the flame in the flame holder zone.
While there are variations on fuel staging, all suffer the disadvantage that they cannot compensate for variations in the airflow to the combustor. Fluctuations in the airflow cause undesirable variations in the fuel/air ratios.
To control the volume of air entering the combustor mixing (primary) zone, it is known to use inlet guide vanes in front of one or more stages of the compressor. Inlet guide vanes reduce the mass of air through the gas turbine. However, inlet guide vanes hamper the capacity of scavenging systems that recover heat for steam production. In addition, inlet guide vanes cannot increase the airflow to the combustor once they are fully open as is common during normal operating conditions. When a gas turbine is operating at full load, the fully open inlet guide vanes are limited to increasing (not decreasing) the fuel/air ratio.
Similarly, air staging schemes have been used to reduce the open area to the dilution plane in a combustor so as to reduce the airflow into the secondary burning area of a combustor. U.S. Pat. No. 4,944,149 discloses an example of air staging. Air staging at the combustor dilution plane suffers the disadvantages of increasing the pressure drop through the combustor by closing openings in the combustors. This pressure drop reduces gas turbine efficiency, especially when the gas turbine is at full load with maximum fuel flow when the diffusion openings are closed.
In addition, air staging has been applied to the open area of the mixing zone (primary zones 3 in FIG. 1A) as is described in K. 0. Smith et al, Development of a Natural Gas-Fired Ultra-Low NOx Can Combustor for an 800 kw Gas Turbine, ASME 19-GT-303 (presented June 1991). In this technique, a large plunger ahead of the inlet tube to the combustor modulates the airflow to the combustor. Since the plunger controls airflow to the mixing zone (but not to the dilution zones), the plunger varies the total open area through the combustor which affects the pressure drop across the combustor. Thus, air staging denigrates gas turbine efficiency by increasing the pressure drop through the combustor. In addition, the plunger appears to have numerous operating positions to complicate its operation.